JPH0564358B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to thermochromic liquid crystal displays. Such displays use selective reflection of cholesteric liquid crystal materials. The observed color depends on the pitch of the cholesteric material and generally changes with temperature. Thermochromic displays have many uses. For example, they are used for temperature measurements such as digital thermometers, medical applications such as vascular system disorder diagnosis, non-destructive tests such as welding flaw inspection, and radiation sensors such as thermal imaging. In all of these applications, display elements have reliability through excellent chemical and photochemical stability and physical robustness, precision through repeatable color calibration, and quality through purity and brightness of the observed colors. There is a need. Some of these requirements are chemical and
For example, British Patent No. 1556994, 1596012,
This problem has been solved by using suitable liquid crystal compounds as disclosed in Nos. 1596013, 1596014 and 1592161. Other requirements are physical and therefore possibly related to the liquid crystal material. The most well-known devices have a liquid crystal material disposed on a supporting substrate. The ink is formulated and printed onto the substrate. This ink contains droplets of liquid crystal material. Two known methods of ink formation use microencapsulation and polymer dispersion. Microencapsulation technology is covered by US Patent No. 3585381
Disclosed in the issue. This technology uses gelatin-gum arabic-based, polyvinyl alcohol-based,
Liquid crystal droplets (5-50 μm in diameter) are encapsulated in a polymer sheet formed from one or a combination of zeene-based systems or amnoblast condensates such as resorcinol-formaldehyde. Suspending these capsules in a water/polymer solution yields an "ink." The polymer dispersion technique is similar to the previous technique, except that the liquid crystal droplets dispersed in the binder polymer film are not initially precoated with a polymer shell. The droplets are placed in cavities within the continuous polymeric matrix layer. U.S. Patent No. 1161039 and
No. 3,872,050 discloses two different methods for producing polymer dispersions. There are problems with both microencapsulation and polymer dispersion. Since only a portion of the liquid crystal material is arranged in an optimally oriented cholesteric helix for optimal reflection, the purity and brightness of the observed colors are partially low. Liquid crystal materials that can change color without encapsulation have been proposed. For example, U.S. Pat. No. 4,251,137 discloses a liquid crystal cell in which a layer of nematic liquid crystal material is contained between the cell walls. The inner surfaces of these walls are formed in a fine rectangular wave pattern, ie, a light grid. When an electric field is applied across the layer thickness, the refractive index of the liquid crystal changes from the refractive index of the zero voltage state.
Two different colors are observed depending on the on or off voltage state of the liquid crystal material. The wall surface shape determines the observed color. This element is not thermochromic,
Therefore, the color does not change with temperature. According to the present invention, the problem of color purity and low reflectance is solved by incorporating a layer of cholesteric liquid crystal material with short pitch between two surfaces, at least one of which has a fine lattice surface geometry oriented with liquid crystal molecules. . According to the invention, there is provided a two-walled thermochromic liquid crystal display comprising a layer of cholesteric liquid crystal material, said liquid crystal material having a cholesteric pitch of between 0.2 μm and 0.7 μm; The pitch value changes with the temperature of the liquid crystal material, thereby selectively reflecting different colors depending on the temperature, and at least one of the walls has a diameter of 0.05 μm.
having a surface formed as a series of gratings and raised portions with a groove width of between 12 μm, so that the molecules of the liquid crystal material in contact with the walls are
A thermochromic liquid crystal display is provided, characterized in that it is uniformly oriented by the grooves. The walls may be flexible or non-flexible, thick or thin, or combinations thereof. Short pitches of cholesteric material are defined as pits suitable for selectively reflecting light, such as preferably pitches of 0.2 μm to 0.7 μm or more. Preferably, the thickness of the liquid crystal material layer is 10 μm or more,
For example, it is 10-5 μm. The two walls have surface gratings that are equal or different in pitch and/or shape. Alternatively, one wall has a grid and the other wall is untreated or treated for homeotropic orientation. The surface grid may be formed by embossing a thin sheet of plastic material. The grating may be rectangular, square, sawtooth, triangular, trapezoidal, sinusoidal, or sufficiently approximate the above shape to form a series of closely spaced alignment grooves. The display can be activated by reflection if the rear wall is coated with an absorbing layer or is designed as an absorbing layer. Alternatively, the displays may be arranged to operate as a narrowband transmission filter by arranging multiple displays optically in series. An enhanced reflective display can be formed by placing a half wave plate between the two displays and providing the rear display with an absorbing rear surface. Memory effects can be incorporated into the display by creating very strong surface orientations that can prevent molecular reorientation after removal of localized heating. At this time, the return to the initial state can be achieved by local shaping, for example by bending. The display may be formed in large sheets joined at the edges and separated by spacer pillars formed on the plastic embossed sheet. Alternatively, the display may be of various shapes, large or small diameter, and may be stamped from a larger display, the stamping being such that the edges can be heat sealed. The particular liquid crystal material used is selected to provide the desired color at a given temperature. This can be accomplished by mixing materials to obtain the desired pitch and index of refraction at a given temperature. The wavelength (λ) of the reflected light is proportional to the average refractive index (n ₀ + _ne )/2 and the cholesteric pitch p. Pitch p changes with temperature. The invention will now be described by way of example with reference to the accompanying drawings, in which: FIG. As shown in FIGS. 1 and 2, a thermochromic display cell 1 has a thin flexible front wall 2 joined at an edge 4 to enclose a short pitch cholesteric liquid crystal material 5. and a rear wall 2. At the rear of the rear wall 3, a black absorption layer 6 made of, for example, Gold Black is formed, or the wall itself is made of, for example, black.
Constructed from a black opaque material consisting of Melinex.
Both walls 2, 3 are made of a 30 μm thick transparent plastic sheet, for example a polyester co-pressed film with an embossable amorphous surface layer.
It can be Melinex 301 (ICI Materials). The thickness of the amorphous layer depends on the dimensions of the desired shape.
For example, the layer thickness can be 3 μm or more. The rear wall 3 of the cell has a grid 7 as clearly shown in FIG.
has been embossed. The grid 7 has a square or rectangular cross-section and typically has a groove width of 0.05 to 12 μm and a depth of 0.05 to 0.1 μm. In the illustrated example, only the rear wall 3 is embossed. The front wall 2 is flat and untreated, but may be embossed with a grating oriented non-parallel to the rear grating. Alternatively, the front wall 2 can be treated, for example, by dipping in lecithin to create a homeotropic orientation. The cholesteric liquid crystal material 5 is oriented by the grating 7 with the helical axis of the director perpendicular to the walls 2,3. Thus substantially all liquid crystal molecules are aligned in an ideal orientation for maximum reflection of a single wavelength. The liquid crystal material 5 has a pitch p and selectively reflects visible light having a wavelength of 3800 to 7800 Å, for example.
Examples of suitable materials include BDH Catalog Number TM
74A, TM 74B, TM 75A and TM 75B.
These materials are mixed to obtain the desired color and temperature range. An example is a mixture that provides a red-blue color effect in the temperature range from 21 to 25.5°C. This is Mixture A = 30% by weight of TM 74A + TM 75A
70% by weight, mixture B = TM 74B 30% by weight + TM
When 75B is 70% by weight, it is formed by 40% by weight of mixture A + 60% by weight of mixture B. mixture A,
Fine adjustment of range and color is possible by adding small amounts of B, TM 74A or TM 75B. The liquid crystal material is introduced between the walls by spreading a thin layer, for example 10 μm thick, onto the shaped surface of one wall. The other wall is coated with liquid crystal and sealed edges, for example by shape punching or machining and heating to, for example, 100°C. A preferred mixed material is the chiral nematic liquid crystal shown below.

[Table] ｜
_CH3

【table】

[Table] Grating 7 is formed on Meline (ICI) by embossing. A copper plate or roller is coated on the nickel plated surface. The plating is then selectively removed using standard photolithographic techniques to create a square or rectangular shape. Preferably, the groove has a width of 0.05 to 12 μm and a depth of 0.05 to 12 μm.
The nickel layer is coated with a thin layer of less than 1 μm release agent such as Fluorad Surfectant FC 721. The plate or roller is heated to approximately 105-115°C;
Pressed onto the amorphous surface layer of Melinex 301 sheet. Typical pressures are 3 to 5 atmospheres. In this way, a fine lattice is formed on Melinex301. During operation, white light is incident on the front wall of the cell. Colored light, for example red in one direction of circularly polarized light, is reflected towards the observer 8 . The same red colored light with opposite direction of circular polarization is transmitted through the two walls 2, 3 of the well and absorbed into the absorption layer 6. Light of other wavelengths passes through the cell 1 and is absorbed by the absorption layer 6. The actual color of the reflected light is determined by the cholesteric pitch p and the refractive index n, where wavelength=np.
When the temperature of the cholesteric changes, the pitch changes and different colors are observed. The amount and range of frequencies reflected, and hence color purity, is enhanced by accurately orienting the cholesteric axis across the display. FIG. 3 shows the surface shape of the wall of cell 1 used in larger displays. The wall 3 is embossed to form a grid 7 and pillars 9 similar to FIG. The pillars make the thickness of the complete cell 1 uniform. In general, pillars have a square cross section of 25 μm x 25 μm, and are 25 μm high, with a height of 250 ~
They are spaced at intervals of 300 μm. embossed
The two pieces of Melinex can be brought into contact and rapidly heated to fuse the pillars of one sheet to an adjacent sheet. The cell walls 2, 3 may likewise be formed from thicker sheets of material, such as for example galaca, but then temperature changes are slow and the cells are less flexible. Another example of the structure of the display is shown in FIGS. 4 and 5. The thin front wall 11 has a fine grating embossed on its inner surface. For example, the grating has a square or sinusoidal shape with a pitch of less than 5 μm, and the grooves, for example, have a width of less than 2.5 μm and a depth of less than 0.1 μm. The rear wall 13 is embossed with rough features 14.
For example, the groove portion 15 has a width of 100 μm and a depth of 10 to 50 μm.
The gap or web 16 between the grooves is 25 μm.
The grooves in the front and rear walls may be arranged parallel or non-parallel to each other, for example perpendicularly. An absorbent layer 6 covers the rear part of the rear wall. As shown in Figure 5, the coarse grid may include grooves 15a extending the entire length of the sheet. Alternatively, the groove can be provided with a cross web 17 to form an elongated 15b or square 15c recess in the surface of the wall 13. The recess may have curved corners and may have an elliptical or even circular planar shape. To form the display cells, the liquid crystal material is applied, for example, by silk screen printing, roller, inkjet printing, or spraying to a depth of approximately 10 μm.
It is spread onto a large sheet 13 with a coarse grid. A large sheet-like front wall 11 is placed on top of the rear wall 13 and both walls are pressed together by heating, for example at 100 DEG C., to seal the web against the front wall. If the coarse grid has cross webs 17, a plurality of closed cells is obtained. The completed display is then cut into shapes using a knife or scissors. Different compositions of liquid crystal material can be printed on different areas of the back wall. These different compositions may be separated or depleted by regions of non-liquid crystal material. The grooves 15 of the coarse lattice can be superimposed on the fine lattice at the bottom. Alternatively, the grooves may be subjected to homeotropic or homogeneous surface alignment treatment. By changing the shape of the grating 7 a memory effect can be created. For example, the depth and/or spacing of the grooves may be reduced to reduce orientation of liquid crystal molecules. Therefore, changes in molecular arrangement caused by local heating are maintained by cooling. Local deformation is necessary to return the display to its initial state. One application of the display of the present invention is a thermal imaging system screen where a thermal image is focused. A large number of microscopic display disks are deposited on a thin support with low lateral thermal conductivity. The use of separated disks in this manner creates a network that reduces the lateral width of the thermal image. Temperature changes across the screen are observed as separate colors representing a thermal image. As mentioned above, the thermochromic liquid crystal display of the present invention is made of a cholesteric material with a short pitch of 0.2 μm to 0.7 μm and is enclosed in a wall whose surface is formed in a lattice shape with a groove width of 0.05 μm to 12 μm. It has superior purity and brightness compared to conventional inks, and can be manufactured at low cost because no special ink manufacturing technology is used.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of a thermochromic liquid crystal display.
Figure 2 is an enlarged view of one wall of Figure 1 showing the internal surface of the shape; Figure 3 is a cross section of another wall useful for larger displays; Figure 4 shows only two walls. 5 is a cross-sectional view of another display shown, and FIG. 5 is a perspective view of one wall of the display of FIG. 1... Display cell, 2, 11... Front wall, 3,
13... Rear wall, 5... Liquid crystal material, 6... Absorption layer, 7... Grid, 8... Observer, 9... Pillar, 15... Groove, 16
...web, 17...crossweb.

Claims (1)

Claims: 1. A thermochromic liquid crystal display having two walls comprising a layer of cholesteric liquid crystal material, said liquid crystal material having a cholesteric pitch between 0.2 μm and 0.7 μm. and the pitch value changes with the temperature of the liquid crystal material, thereby selectively reflecting different colors depending on the temperature, and at least one of the walls has a groove width between 0.05 μm and 12 μm. thermochromic, characterized in that it has a surface formed as a series of lattices and raised portions, whereby the molecules of said liquid crystal material contacting said walls are uniformly oriented by said grooves; LCD display. 2. The display according to claim 1, wherein the surface shape of one wall is different from the surface shape of the other wall. 3. The display according to claim 1, wherein one wall is surface-treated in a fine lattice pattern, and the other wall is surface-treated in a coarse lattice pattern. 4. A display as claimed in claim 1, wherein the coarse grid comprises cross-webs such that the liquid crystal material is contained within closed cells. 5. The display according to claim 1, wherein a light absorbing material is disposed at the rear of the display. 6. The display according to claim 1, wherein the wall is thin and flexible. 7. A display as claimed in claim 1, wherein one wall is treated to form a homeotropic alignment with the liquid crystal material it contacts. 8. A display according to claim 1, which is formed from two display elements with a half-wave plate between them, and a light absorbing material is arranged at the rear of the display. 9. A display according to claim 1, wherein each area of the display contains a different composition of cholesteric liquid crystal material.